This invention is directed to test slides for use in a vision testing apparatus. Specifically, a test slide includes component layers designed to reduce the occurrence of interference fringes that can disrupt the effectiveness of the vision testing apparatus.
Vision testing equipment commonly includes test slides that display the visual optotypes that are designed to evaluate various vision abilities. The test slides may be permanently fixed in an apparatus. Alternatively, the test slides may be removably mounted in the apparatus to allow for various types of tests to be performed by varying the size, orientation, shape, color, etc. of the optotype being displayed. This removable mounting technique allows for custom selection of slides in a given testing apparatus. It also allows for the replacement of slides if they become faded, distorted, or otherwise no longer effectively operable.
There are many types of test slides currently in use. Some slides are made up of a monolayer of film generally similar to projection slide systems. Some slides have optotypes directly printed onto a single glass layer. Most slides, however, are multiple layer, laminated assemblies. These layers may include a light diffusing layer, a color correction layer, an optotype layer, a clear glass cover layer, or other types of layers. These layers in various combinations may be used by different slide makers for different applications.
A potential problem that arises when laminating transparent layers together, particularly relatively thin layers as is common in test slides, is the appearance of interference fringes (also called Newton rings) when looking at the slide. Simply described, interference fringes are the xe2x80x9crainbowsxe2x80x9d that are visible on the slide that are the result of imperfect flatness (or imperfect lamination) of adjacent thin transparent layers, particularly a thin film next to flat glass plate. The light and dark rings are produced by the interference of light at the film of air between the surfaces. The rainbows that result from this physical phenomena can be distractive and, possibly, disruptive of the vision test that is being performed. Even if the slides do not have a significant rainbow effect when new, rainbows can develop and get worse over time after the slide is in the apparatus.
Historically, in order to address the issue of interference fringes or rainbows, slide manufacturers have required very tight specifications regarding the flatness of the various layers that comprise the slide. These strict specifications mean that the components are typically relatively expensive. Alternatively, the layers have been separated into different slides, but this solution is limited by the space that is available in the apparatus itself. Another attempted solution is coating of glass to achieve, for instance, color correction in order to reduce the number of layers. This solution, however, has many of the foregoing drawbacks with respect to flatness.
Accordingly, it is an objection of the present invention to overcome the foregoing drawbacks and provide a test slide having reduced or no interference fringes. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
In one embodiment, the invention includes a test slide for use in a vision testing apparatus comprising several component layers. The layers include a light diffusing layer, an optotype film layer, a colored film layer, a frame separator and a clear glass layer. The components are laminated together in the order identified. The laminated slide may further comprise an opaque tape for securing the edges of the components together to form the laminate slide.
In a further embodiment, a test slide is used in a vision testing apparatus, the slide comprising several component layers. Those component layers include a clear glass layer, a frame separator laminated adjacent to the clear glass layer, and an optotype film layer.
In a still further embodiment, a test slide is designed for use in a vision testing apparatus wherein the apparatus comprises a light source and a viewer. The test slide is mounted between the light source and the viewer, and the test slide has a first side facing the light source and a second side facing the viewer. The test slide comprises an optotype film layer and a colored film layer wherein the colored film layer is on the side of the optotype film layer facing the viewer.
Another embodiment includes a test slide for use in a vision testing apparatus. The test slide is comprised of a lamination of a plurality of layers wherein a frame separator is one of the layers.